Subsea flowlines are used to transport fluids produced at offshore gas/oil wells to a production platform located some distance (offset) away from the wells. The flowlines typically contain a mixture of oil, natural gas, and water. The mixture is at a relatively warm temperature when it is first extracted from the subterranean reservoir, but it cools as it moves through the flowline from the wells to the production platform. Indeed, the cooling rate becomes quite rapid if the production flow stops, and this occurrence is termed in the industry as "shut-in."
If the mixture cools too much, two transformations can occur. Hydrates can begin to form at sufficiently low temperatures when natural gas and water combine into an ice-like structure. Wax deposition on the flowline walls begins when the flowline pipe walls cool below the cloud point of the oil.
To combat these problems, some subsea flowlines have been insulated to minimize steady-state heat loss through the pipe walls while fossil fuel mixtures are flowing through them as well as when the flow has been stopped (i.e., the shut-in condition). There are several commercially available insulation materials for application to subsea flowlines used to transport such fluid mixtures from offshore gas/oil wells. These available insulation schemes include both non-jacketed and pipe-in-pipe designs.
Non-jacketed insulations are coated directly on the exterior of a pipe. Pipe-in-pipe insulation systems have an insulating medium contained within an annular region between an inner pipe (the fluid carrier) and an outer pipe (the jacket). Conventional insulation used as the insulating medium prolongs the cooling time for the pipe contents during a shut-in condition by both reducing the heat loss rate to the ambient seawater and by providing some additional sensible thermal storage capacity. However, the sensible heat storage capacity of conventional insulation is not very large.
The joints between pipe-in-pipe flowline sections are particularly susceptible to releasing heat from the flowline contents during shut-in conditions. During flowing conditions, the joints represent a small fraction of the total surface that the flowline contents contact. However, during shut-in conditions, the fluid mixture of components adjacent the joint cool much faster than those in the rest of the flowline pipe.
Several methods and apparatus have been applied to pipelines to reduce the problems associated with cooling of the flowline mixture. These include passive insulation schemes, as discussed above, active heating approaches which use electrical or other heating mechanisms, and the use of chemical inhibitors to lower the melting points of the mixture components or otherwise prevent them from forming blockages. Active heating and chemical inhibitors have been successfully used to prevent wax and hydrate formation in both flowing and shut-in conditions. Passive insulators, however, do not adequately extend the time before waxes and hydrates form in cases where ambient conditions are very cold or normal operating temperatures are not far above critical temperatures.
Phase change materials have been studied for many years, with research reaching a peak in the 1970's and 1980's. It is well known that all materials exhibit phase changes in their physical form as they pass from solid to liquid, and from liquid to gas, as heat energy is added. At their phase change temperatures, all materials absorb (or release) energy while remaining at a relatively constant temperature. The heat energy absorbed or released during phase changes is called the latent heat of the material. The latent heat can be used as a thermal energy storage for maintaining a warm or cool temperature in the region adjacent the phase change material.